1. Field of the Invention
This invention relates to transistor oscillator circuits and more particularly to a self-optimizing Hartley oscillator circuit for use with induction heating systems.
2. Prior Art
High frequency induction heating generators conventionally take the form of vacuum tube or semi-conductor oscillators having the primary of the induction heating output transformer forming part or all of the inductance in the oscillator tank circuit. These circuits must include means for stabilizing the oscillator against changes in the inductance of the tank circuit resulting from changes in the coupling of the load to the inductance or from variations in impedance of the load which result in reflected variations in impedance of the tank circuit. A number of techniques have been developed in the prior art to stabilize the oscillator with respect to these changes. Broadly, these techniques have either been to design the tank circuit and the induction heating coil so that the change in load does not appreciably affect either the frequency of oscillation or the Q of the tank circuit or to provide auxilliary impedance elements which would be switched into the tank circuit as the load changes to maintain the oscillator operating conditions at an optimum.
Efforts to minimize the effects of load change on the tank circuit have typically involved forming the tank inductance of a relatively small coil section which acts as the induction heating transformer primary and a substantially larger inductance section which does not have its field coupled to the load. The variations in impedance which occur when the small primary winding section of the tank coil is coupled to the load thus represent a small percentage of the total coil impedance and do not substantially deteriorate the Q of the amplifier or shift its operating frequency. In designing these circuits the portion of the induction coil which acts as the induction heating primary must be selected to strike a balance between the losses in efficiency which are sustained because the large induction heating currents must pass through the non-coupled section of the tank inductance and the loss in stability in face of a varying load, which increases in effect as the proportion of the coil dedicated to inducing currents in the load is increased. As a result, these circuits can often be characterized as having both efficiency and stability which is substantially less than the optimum. The circuits which switch auxilliary impedance elements in the tank circuit to compensate for the impedance variation caused by loading must involve relatively simple, manually actuated switching circuits that depend for their accuracy on a priori calculations as to the effect of the load on the inductance or relatively complicated and expensive self-balancing circuits.
Considering a separate technology that may be deemed prior art to the present invention, previous efforts have been made to stabilize oscillators to provide constant magnitude output pulses against variations in the ambient temperature which vary the impedance and gain of the semi-conductor elements. Some of these oscillators employ bias circuits which shift the operating point of the amplifying element to compensate for temperature variations. For example, U.S. Pat. No. 2,849,611 discloses a regenerative oscillator employing a transducer which varies a DC output of the amplifier as a function of changes in impedance of the transducer. A bias network employing negative temperature coefficient elements compensates for shifts in the operating point produced by the effects of temperature on the semi-conductor amplifier. The shift in bias point produced by circuits of this type is quite small and they operate to maintain the frequency of the oscillator constant independent of the temperature variations.